The present invention relates to a semiconductor device and a receiver.
In wireless communication (wireless link), the frequency accuracy of a wireless device (transmitting and receiving device) is standardized to maintain the accuracy required for guaranteeing satisfactory communication quality. A reference oscillator, which is incorporated in a wireless device, is required to have a high frequency accuracy to enable temperature compensation. However, the wireless standard is relatively moderate (the margin of error allowed for frequencies being relatively broad) for wireless devices used in a short-distance wireless link. As an example, in BLUETOOTH, for each channel having a bandwidth of 1 MHz, an initial frequency accuracy of 75 KHz and a frequency drift of 40 KHz are tolerated. In such a case, the wireless device does not require a highly accurate reference oscillator and thus reduces costs. When a relatively moderate standard is used, a receiver must perform demodulation correctly even when there is an error in the frequency of the signal received from a transmitter.
During wireless communication, if the frequency of the signal received from the receiver is not included in the range of a channel selecting band pass filter (BPF), the received signal is distorted. This increases interference between codes and increases the bit error rate. As a result, the receiver cannot correctly demodulate the receiving signal.
Accordingly, in a conventional receiver, the number of stages in the BPF is increased to prevent a decrease in the receiving capability with respect to frequencies that are not included in a channel. By increasing the number of stages in the BPF, the passing bandwidth is broadened to include predetermined tolerated frequencies. When merely broadening the passage bandwidth of a single BPF, the attenuation outside the channel is reduced. Thus, the increase in the number of stages of the BPF prevents a decrease in the attenuating capability.
Normally, in an FM receiver, the intensity of the receiving signal of the receiver is monitored to vary the oscillation frequency of a local oscillator (LO) in accordance with the signal intensity. This adjusts the intermediate frequency (IF) of the received signal to the center of the BPF passage bandwidth. For example, in a mobile communication terminal, such as a cellular phone, the receiving sensitivity is improved to control the reference frequency of the mobile terminal so that it matches the reference frequency of a base station.
However, when broadening the passage bandwidth of the BPF, the noise resisting capability of the receiver decreases. Thus, the receiving capability cannot be improved. Further, an increase in the number of stages of the BPF increases the circuit area. Since the circuit area of the BPF is especially large, the receiver is enlarged when the number of stages is increased.
The BPF is normally incorporated in the receiver. The frequency of the IF signal used in the receiver is usually set at a low frequency (for example, 1 to 3 MHz for BLUETOOTH) in correspondence with the channel of the BPF. That is, the received signal is converted to the low frequency IF signal to reduce the circuit area of the receiver and lower costs. In such a receiver, the conventional adjustment of the local oscillator frequency in accordance with the intensity of the received signal decreases the detection accuracy near the peak of the signal intensity. Thus, the frequency of the IF signal cannot be controlled accurately. Further, such a receiver cannot cope with fading that occurs in the level of the received signal and cannot be applied to controls that require high speed response, such as time division duplex (TDD).
In a short-distance wireless link in which devices provided with the BLUETOOTH function are connected, every one of the devices may be a master device or a slave device. In such a case, when controlling the reference frequency of the slave device to match the reference frequency of the master device, the receiving capability of the slave device is improved. However, the initial frequency accuracy decreases when the slave device transmits a signal. Such control is not preferable.